Apparatus and method for reducing hydrofoil cavitation

ABSTRACT

A method of reducing underwater hydrofoil cavitation is described; hydrofoil steering vanes trade a small degree of efficiency for significant cavitation reduction, through perforations in the hydrofoil. Some pattern examples are discussed. A method of further cavitation reduction through surface texture variation also reduces underwater hydrofoil drag; some patterns are discussed. Combining the methods provides maximum cavitation reduction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This non-provisional application claims the benefit of U.S. ProvisionalPatent Application Ser. No. 60/417,127, filed Oct. 10, 2002, as well asthe benefit of U.S. patent application Ser. No. 10/164,730 filed Jun. 6,2002, the benefit of U.S. patent application Ser. No. 10/171,273 filedJun. 12, 2002, the benefit of Provisional Application Ser. No.60/297,314 filed Jun. 12, 2001, the benefit of U.S. ProvisionalApplication Ser. No. 60/361,950 filed Mar. 7, 2002, and the benefit ofU.S. patent application Ser. No. 09/718,753 filed on Nov. 22, 2000, nowissued as U.S. Pat. No. 6,427,618 for all that they contain and teach.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to rudders (and other underwaterhydrofoils) for aquatic vessels and more particularly to a rudder (orany submerged hydrofoil) and methods for minimizing early cavitation. Italso has applications for fluid pumps or turbine impellers.

2. Problems in the Art

Rudders provide a vessel with directional stability, control, andmaneuverability. Ship rudders are generally vertical hydrofoils withsymmetrical profiles. Rudders generate hydrodynamic lift forces toproduce ship-turning moments for maneuvering and directional control.The lift force produced varies with the rudder's angle of attack (angleof the rudder relative to the onset flow angle) and the incoming flowvelocity (velocity of flow into the rudder). The effectiveness ofrudders in performing their hydrodynamic functions is proportional tothe square of the incoming flow velocity; thus, rudders are generallyplaced behind propellers where the incoming flow is accelerated by therotating propeller. Marine engineers initially determine the overallrudder size (rudder chord, i.e., longitudinal distance from leading edgeto trailing edge, and rudder span, i.e., vertical distance from root totip) based on consideration of the ship's required turning diameter andthus required turning moment.

These conventional rudders are located either along the ship centerlinedirectly behind the propeller, in the case of a single shaft designs, orbehind the propellers and positioned symmetrically about the ship'slongitudinal centerline in the case of a multiple shaft designs.Therefore, the rudders are located in the propeller slipstream ortrailing wake, i.e., accelerated flow in the propeller wake. Thepropeller slipstream is a region of highly complex flow having axial,tangential and radial flow components. The rotating propelleraccelerates the flow and sheds vortices that impinge on the ruddersurface. Consequently, a rudder operating behind a rotating propellerencounters, in addition to the flow at substantially the ship's velocity(ship wake), vortices shed by and induced velocity generated by thepropeller (propeller's trailing wake).

The simple conventional rudder design practice results in problems interms of rudder performance. Depending on the propeller's size,hydrodynamic loading and position relative to the rudder, the incomingflow to the rudder exhibits induced flow angles (onset flow angles) thatcan vary longitudinally along the chord of the rudder (chord-wise) andvertically along the span of the rudder (span-wise). Because thepropeller accelerates and rotates the flow into the rudder and becausethe vortices shed from the propeller impinge on the rudder surface, theflow entering the rudder plane exhibits larger onset flow angles (anglesof incoming flow relative to the ship longitudinal centerline) thanwould result without the propeller present. As a result of the propellergenerating non-zero onset flow angles in the rudder plane, rudders withsymmetric profile sections placed parallel to the ship centerlineexperience non-zero angles of attack, generate hydrodynamic lift, andincur induced drag; even if the ship is operating in a straight aheadcourse.

Because of non-zero onset flow angles, suction pressure peaks (highlydecreased pressure) occur at or near to the leading edge of the rudder.Surface cavitation can be predicted from the pressure distribution onthe rudder surface. Cavitation inception occurs when local pressuredrops to or below the local vapor pressure of the flowing fluid.Therefore, in areas of suction pressure peaks, early cavitationinception can occur on the rudder outboard side (on a single propeller).

Viewing the rudder and propeller from behind the ship, a right-handrotating (clockwise rotating) propeller will produce flow havingvelocity components directed to the right of the propeller centerlinewhile left-hand rotating (counterclockwise rotating) propellers willdirect components of flow to the left of the propeller centerline.Therefore, depending on the direction of propeller rotation and, thus,the direction of induced flow angle into the rudder, cavitation mayoccur on either the inboard or outboard side of the rudder in twin shaftdesigns.

Early rudder cavitation results in an undesirable compromise inhydrodynamic and acoustic performance of the vessel. Specifically,rudder cavitation induces unsteady hydrodynamic forces, vibration, andrudder erosion. The existence of non-zero onset flow angles also reducesthe available rudder angles for avoiding rudder stall at low speeds.Furthermore, the induced drag from the finite lift force as well as theform drag from the rudder cavity creates additional ship resistance,wasting power and degrading performance.

Ship rudders are subjected to propeller induced velocities and inducedflow angles that vary along the rudder span and chord. Because ofnon-zero onset flow angles, a suction pressure peak is formed at or nearthe leading edge of the rudder where early cavitation occurs. It would,therefore, be advantageous to both hydrodynamic and acousticperformances to alleviate the occurrence of suction pressure peaks andearly cavitation. Thus, there is clearly a need to improve ruddercavitation inception speed (i.e., delay cavitation inception) and toimprove hydrodynamic and acoustic performances of rudders.

Due to the complexity of flow field in the propeller slip-stream, theinfluence of accelerated cross-flow induced by the propeller onto therudder was not considered in existing rudder design practice until U.S.Pat. Nos. 5,415,122, 5,456,200 and 6,101,963 by Shen significantlyimproved the rudder cavitation problem with a “twisted” rudder andrudder tab at the lower end tip.

For example, on the Arleigh Burke Class destroyers, it had becomeapparent that rudder cavitation was a serious problem. Unfortunately,this cavity began to form at speeds as low as 23 knots, which means itis present at most normal vessel operating conditions. This cavitationled to erosion of the protective coating on the rudder and led to highmaintenance costs for the fleet. To alleviate this phenomenon, Dr. YoungT. Shen's rudder design more closely matched the complex flow in thewake of the propeller. By “twisting” the rudder section around thestock, he was able to develop a rudder that would not cavitate undernormal loading conditions. In addition, by developing a tip plate forthe end of the rudder, he was also able to greatly reduce the tipcavitation from the standard fleet design. For his work, Shen wasawarded U.S. patents on both the twisted rudder and tip plateinnovations. Models of the twisted rudder were tested in a largecavitation channel and compared to the standard fleet rudder. Thetwisted rudder showed a significant improvement over the fleet design,not experiencing cavitation until it approached nearly 30 knots.

If cavitation could be reduced even further by an alternative method,this alternative method would have applications at even higher speeds,such as found on high-speed ferries. At lower speeds, any rudderslightly out of perfect alignment may also experience cavitation, untilperfect alignment is achieved. Rudders are often out of perfectstraight-ahead alignment. There is a need for minimizing cavitation inwaterjet underwater steering plates, for which “twisting” will notcorrect. Cavitation reduction is even more important in submarinestealth. Submarine rudders are not commonly found in the propellerslipstream; however, minimizing any cavitation in the hydrofoil steeringplanes is very desirable. There is therefore a need to reduce ruddercavitation even further on any hydrofoil steering system.

FEATURES OF THE INVENTION

A general feature of the present invention is the provision of a methodor methods for maneuvering a surface or submarine ship through water,which overcomes problems found in the prior art.

A further feature of the present invention is the combination of thetwisted rudder (in the slipstream) with the new features providedherein.

Another feature of the present invention is the combination of the tipplate with the new features provided herein.

Still another feature of the present invention is the provision of amethod (or methods) for reducing rudder cavitation on a straight rudder,twisted rudder or submerged hydrofoil steering plane, with or withouttip plates.

Still yet another feature of the present invention is the provision of amethod (or methods) for reducing cavitation on any hydrofoil, includingfluid pumps and turbine impellers.

Another feature of the present invention is the provision of a(rudder/steering plane) cavitation reduction process that incorporateseither hydrofoil perforations or surface texture variation individually,or both in combination.

A further feature of the present invention is the provision of anapparatus that is lighter in weight, yet as strong as the rudder that isreplaced.

A still further feature of the present invention is the provision of amethod and apparatus for steering a surface ship or submarine throughwater that is more efficient at high speed, saving fuel by loweringdrag.

Yet another feature of the present invention is the provision of amethod and apparatus for minimizing cavitation damage.

These, as well as other features and advantages of the present inventionwill become apparent from the following specifications and claims.

BRIEF SUMMARY OF THE INVENTION

Rudders are hydrofoils, as are propeller blades. Technology that reducescavitation in one will reduce cavitation in the other. U.S. patentapplication Ser. No. 10/164,730 filed Jun. 6, 2002, and U.S. patentapplication Ser. No. 10/171,273 filed Jun. 12, 2002, by Hilleman,discuss perforated propellers to reduce cavitation for marinepropulsion. In the case of a perforated rudder, the same principlesapply; it is understood that perforations must not compromise structuralstrength.

The less efficient perforated propeller is a trade-off for lesscavitation. Increasing propeller pitch also trades efficiency forreduced cavitation; this latter trade-off is well known in the art.Cavitation reduction is advantageous for stealth. In the perforatedpropeller application, the holes should have smooth rounded edges andshould be large enough to avoid biologic blockage, yet be small enoughto interfere with efficiency as little as possible. They should belocated wherever cavitation is a problem, e.g. ahead of the pitting toavoid cavitation inception. The same may be generally said of theperforated rudder (or any submerged hydrofoil); however, efficiency maybe less of a concern, while strength may be more of a concern. A rudderwith staggered rounded perforations is nearly as strong as a solidrudder. Any perforation pattern would apply; some examples would bestaggered round holes, staggered teardrops, or parallel longitudinalslits running the length of the chord. The longitudinal slits runningthe length of the chord reduce boundary layer turbulence (as well ascavitation) extremely well. All have the advantage of markedly reducingboundary layer turbulence as well as controlling cavitation on thesuction side; they also reduces weight without significantly affectingperformance strength.

The above Hilleman patent applications also discuss surface texturevariation to minimize boundary layer drag. Boundary layer drag can besignificant in turbulent water; it is a significant cause of submarinecavitation. Frontal wave drag is the primary resistance of surfaceships. When both are combined, they essentially comprise the surfaceship's drag force in the water while underway. On a well-trimmed rudder,these drag forces act on the rudder; they worsen as rudder alignmentworsens. Rudder alignment is not always perfect; therefore, streamliningthe rudder surface will help to minimize drag (and decrease cavitation)at all speeds, particularly in turns.

Surface drag is due to viscous shear forces of the moving water againstthe surface of the ship and rudder, resulting in eddies and turbulencethat cause deceleration, sapping the ship and rudder's momentum.

The turbulence and eddies increase with increase in passing water speed.Parallel longitudinal ridges, like those found on a phonograph record,would allow the water to flow as close to the surface as possible,without touching it, thereby reducing the turbulence close to thesurface. For example, 40 micron phonograph-like ridges covering thegreater surface area of the rudder would create a shear-protected layerof similar magnitude, preventing eddies of high-speed fluid fromcontacting the surface. This would streamline the pressured side of therudder, reducing drag. A similar texturing resembling fine sharkskinscales or tiny dimples would also have a similar benefit.

Suction pressure peaks (highly decreased pressure) and associatedcavitation often occur at or near to the leading edge of the rudder.Incorporating coarser texturing of larger sized ridges at the rudder'sleading edge, that eventually blend into the finer ridges on main bodyportion of the rudder, helps reduce cavitation on the leading edge'ssuction side. The coarser ridges act like golf ball dimples to drawpressurized water closer to the low-pressure surface, much as golf balldimples pressurize the air behind the golf ball. There are differences,since air is compressible; however, the fluid principles are similar.Drag on a golf ball comes mainly from air-pressure forces. The dragarises when the pressure in front of the ball is significantly higherthan the pressure behind the ball. The only practical way of reducingthis differential is to design the ball so that the main stream of airflowing by it is as close to the surface as possible.

The dimples augment the turbulence very close to the surface, bringingthe high-speed air stream closer, causing it to travel further aroundthe ball before separating and “tucking it in” behind the round ball toa much greater degree. Reducing the “low pressure wake” generated thusincreases the pressure behind the ball, which reduces the pressuredifferential, as well as it reduces the vacuum magnitude, and thereforeresults in an overall drag reduction. This drag reduction allows thedimpled golf ball to fly about two and one half times farther than anidentical but undimpled ball. The faster the undimpled ball moves, themore the drag is increased; however speed does not affect drag very muchon a ball with dimples.

Water, being incompressible, has suction turbulence rather than a vacuumpressure; suction turbulence fosters cavitation. Minimizing suctionturbulence, by variable texturing (coarser ridges or golf-ball dimples)on the rudder's leading edge, will minimize cavitation on the rudder'sleading edge. In addition to coarser ridges and golf-ball dimples, othertextures, such as large fish scale patterns may be used to accomplishthe same effect. Mammal-like (Cetacea) polymer coatings or fluid-backedrubber coatings could also be utilized in a similar variable role on theleading edge (as well as on the main body).

Suction turbulence is present at the trailing edge. The trailing edge isoften rounded to minimize cavitation, as a sharp trailing edge can leadto cavitation at lower speeds. The golf-ball dimple effect, the coarserlongitudinal ridges, or the larger fish scale texturing would be ofgreat advantage in this location, as they would pressurize the roundedtrailing edge. In addition, the golf ball dimpling effect could beincorporated in Dr. Shen's tip plate design, for example, dimpling therounded rudder tab at the lower end tip would pressurize a greaterportion of it's rounded trailing end.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial side view with evenly spaced round perforationsthroughout the body of the rudder.

FIG. 2 is a pictorial bottom (or top) view of the rudder illustrating agolf-ball dimple pattern on the rounded corners and edges.

FIG. 3 is a pictorial view of some of the different surface texturesthat could be used in the variable rudder texturing.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described as it applies to its preferredembodiment. It is not intended that the present invention be limited tothe described embodiment. It is intended that the invention cover allmodifications and alternatives, which may be included within the spiritand scope of the invention.

Referring to FIG. 1, FIG. 1A describes an embodiment in which the entirerudder 10 is covered by alternating round-hole perforations 11, asmagnified in 1B. The technique of drilling holes is well known in theart. In this case the rounded perforations would result in a dimpledsurface (FIG. 3A), capable of evenly pressurizing the suction side aswell as augmenting surface turbulence and reducing boundary layer dragon the pressure side (from water passage through the holes).

In the FIG. 2 bottom rudder view, this embodiment illustrates surfacetexture minimizing leading edge cavitation and trailing edge cavitation,without perforations. Rudder 10 has a leading edge 12 and a trailingedge 14, moving in the direction of arrow 16. FIGS. 2A and 2B illustrategolf ball dimple pattern on the rounded leading and trailing edges.Because they cover all rounded edges, they always cause the water toflow more around and behind the rounded dimpled surface more than thatseen on a smooth rounded surface. This reduces the area and intensity ofsuction turbulence, which in turn reduces cavitation on the leading edgesuction-prone area 13 and trailing edge suction-prone area 15.

FIG. 3A better illustrates the golf ball dimple surface texturetreatment shown in FIG. 2. If it were to be used on a non-perforatedrudder, the dimples 11 on the main body of rudder 10 would be muchsmaller than the heavy pronounced dimples in the rounded leading 12 andtrailing edges 14. These small dimples would more closely approximatethe size of the longitudinal phonograph ridges previously mentioned. Thelongitudinal ridges could be used in another embodiment, as could theshingle pattern of 3B or the shark or fish scale pattern of 3C. Thesepatterns could be impressed in the rubber coating during molding. Ineach case, smaller texturing is used to reduce drag on the pressure sideof the rudder 10 main body, and coarser texturing is utilized on thecurved surfaces to augment surface turbulence and carry water pressurearound the curve, pressurizing the back side (13&15) more.

Maximum benefit is possible through a combination of perforations andvariable texturing. Combining FIG. 1 and FIG. 2 would be an example ofsuch an embodiment. The combination of perforations could be utilized onany submerged hydrofoil to reduce cavitation, regardless of whether itis a propeller, pump impeller, turbine impeller, waterjet steeringplate, lifting hydrofoil or submarine steering plane. It is to befurther understood that the hydrofoil system is dynamically designedaccording to desired performance characteristics.

This is therefore believed to have accomplished all of the statedobjectives of the invention including the provision of a method ormethods for maneuvering a surface or submarine ship through water, whichovercomes problems found in the prior art; the combination of thetwisted rudder (in the slipstream) with the new features providedherein; the combination of the tip plate with the new features providedherein; the provision of a method (or methods) for reducing ruddercavitation on a straight rudder, twisted rudder or submerged hydrofoilsteering plane, with or without tip plates; the provision of a method(or methods) for reducing cavitation on any hydrofoil; the provision ofa (rudder/steering plane) cavitation reduction process that incorporateseither hydrofoil perforations or surface texture variation individually,or both in combination; the provision of an apparatus that is lighter inweight yet as strong as the rudder that is replaced; the provision of amethod and apparatus for steering a surface ship or submarine throughwater that is more efficient at high speed, saving fuel by loweringdrag; and the provision of a method for minimizing cavitation damage.Accordingly, all such modifications and additions are deemed to bewithin the scope of the invention, which is to be limited only by thefollowing claims.

1. A method of reducing hydrofoil cavitation on a ship, the methodcomprising: providing perforations in a hydrofoil, the hydrofoil havinga leading edge, a trailing edge, a suction side and a pressure side, theperforations providing a passage for water from the suction side to thepressure side, and maneuvering the ship through water.
 2. A hydrofoilthat reduces cavitation, the hydrofoil comprising: a main body entirelycovered by alternating round-hole perforations capable of evenlypressurizing a suction side as well as augmenting surface turbulence andreducing boundary layer drag on a pressure side; a trailing edge on themain body; and a surface texturing on the trailing edge.
 3. Thehydrofoil of claim 2 further comprising a leading edge on the main bodyand surface texturing on the leading edge.
 4. The hydrofoil of claim 2wherein the surface texturing comprises dimples.
 5. The hydrofoil ofclaim 2 wherein the surface texturing comprises longitudinal grooves. 6.The hydrofoil of claim 2 wherein the surface texturing comprises scales.7. The hydrofoil of claim 2 wherein the surface texturing comprisesshingles.
 8. A hydrofoil that reduces cavitation, the hydrofoilcomprising: a main body having a leading edge, a trailing edge, asuction side and a pressure side; and perforations in the main bodyproviding a passage for water from the suction side to the pressureside.